This invention relates to variable line equalizers and line build-out networks (LBO's) for subscriber carrier transmission systems and more particularly to such circuitry employing a pair of complementary constant-resistance networks (CRN's) that are terminated by differentially controlled variable resistors.
The transmission of voice signals over subscriber carrier circuits requires that the transmission characteristic of a channel, including the communication medium which may be a twisted-pair telephone cable, be sufficiently uniform that there is a minimum distortion of information being transmitted. It is well known, however, that the attenuation characteristics of a telephone cable change with signal frequency and the length of a line. It is necessary therefore to employ equalizers and/or line build-out networks to equalize the channel by introducing certain amounts of attenuation in selected portions of the signal frequency spectrum to compensate for the nonuniform attenuation introduced by the cable pair. Stated differently, the attenuation characteristic of the equalizer must be complementary to that of the unequalized channel so that the sum of decibel measures of these attenuations is approximately constant with frequency, i.e., so that the resultant attenuation characteristic in the channel is flat. Although this means that the resultant signal level is reduced, this loss in signal can be compensated for in an amplifier.
Various types of equalizers and line build-out networks have been employed for accomplishing this function. Bode-type equalizers are three-port networks which are often employed in these applications. They are described in U.S. Pat. No. 2,096,027 by H. W. Bode, filed Jan. 30, 1936 and issued Oct. 19, 1937, and the article, "Variable Equalizers" by H. W. Bode, pp. 229- 244, The Bell System Technical Journal, vol. 17, No. 2, April 1938. Although such equalizers often employ only passive elements and a single resistor as a variable impedance element, active Bode-type equalizers are also available as are such networks employing the combination of a constant resistance network (CRN) and a variable resistor as the variable impedance element. CRN's are also employed as variable equalizers. Such networks are generally described in the Bode U.S. Pat. No. 2,096,027 and in the articles, "Distortion Correction in Electrical Circuits with Constant Resistance Recurrent Networks" and "Theory and Design of Uniform and Composite Electric Wave Filters", both by Otto J. Zobel, The Bell System Technical Journal, pp. 438- 534, vol. 7, No. 3, July 1928 and pp. 1- 46, vol. 2, No. 1, January 1923, respectively. A CRN is essentially a two-port network which is comprised of resistors and one or more pairs of inverse networks of constant impedance product. In a CRN having one port terminated by a resistance R.sub.o which is equal to this product, the image impedance across the other port is also equal to R.sub.o and is constant over a frequency band where the terminating resistance remains constant and equal to R.sub.o. In a CRN of appropriate design, its attenuation characteristic is complementary to that of a cable pair or other type of transmission medium when the former is terminated by a resistance other than R.sub.o that is related to the line length. It is known that the variable terminating resistance of the CRN may be produced by an electronically controlled element such as a thermistor. Such networks provide a limited range of equalization. A different type of equalizer is described in the U.S. Pat. No. 3,806,839 by Takuya Iwakami, filed Dec. 12, 1972 and issued Apr. 23, 1974.